From 17cf6b6fa02db452c3b0b88b09b8884f73b0c1eb Mon Sep 17 00:00:00 2001
From: pommicket <pommicket@gmail.com>
Date: Wed, 17 Nov 2021 22:58:17 -0500
Subject: 04a readme and corrections

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+# stage 04a
+
+Rather than a compiler, this stage only consists of a simple [preprocessor](https://en.wikipedia.org/wiki/Preprocessor).
+In the future, we'll run our code through this program, then run its output
+through a compiler.
+
+It take lines like:
+```
+#define THREE d3
+```
+and then replaces `THREE` anywhere in the rest of the code with `d3`.
+I've provided `in04a` as a little example.
+Unlike previous programs, you can control the input and output file names
+without recompiling it. So to compile the example program:
+```
+make out03
+./out03 in04a out04a
+```
+
+Although it seems simple, this program will be very useful:
+it'll let us define constants and it'll work in any language.
+There really isn't much else to say about this program. With that,
+we can move on to [the next stage](../04b/README.md) which should be more exciting.
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